1. Field of the Invention
This invention relates to an apparatus for measuring the line width of a pattern formed on a substrate. More particularly, it relates to such an apparatus for measuring the line width of a pattern on a substrate by the use of coherent light diffracted by the line edges of the pattern.
2. Description of the Prior Art
In the process of forming an IC pattern on a semi-conductor wafer, it is very important to determine, by accurate measurement whether or not the line width of a pattern on the wafer is exactly formed to an intended width, in order to ensure the electrical performance of the IC chip ultimately obtained. Therefore, the present invention will hereinafter be described with respect to a device for measuring the line width of an IC pattern formed on a semiconductor wafer (hereinafter referred to as a chip pattern), although the invention is not restricted to the measurement of the line width of a chip pattern but is widely applicable to the measurement of the width of other patterns having such properties as will hereinafter be described.
The conventional method of automatically measuring the line width of a chip pattern will generally be explained by reference to FIGS. 1 and 2 of the accompanying drawings. In FIG. 1, a light source for illumination 1, a condenser lens 2, a half-mirror 3 and a microscopic objective lens 4 together constitute a microscopic projecting illumination system. A semiconductor wafer 5 is set on a base or stage 8 which is movable in the direction of arrow X. A chip pattern 6 is formed on the semiconductor wafer 5, and the enlarged optical image of the chip pattern 6 may be focused through a slit 9 by the objective lens 4 of the microscope. As the movable stage 8 is moved in the direction of arrow X, the variation in the quantity of light passed through the slit 9 may be converted into an electrical signal 12 by a photoelectric conversion element 11, whereby there may be obtained the output as indicated by 12 in FIG. 2. As seen in FIG. 2, the electrical signal 12 depends on the optical contrast of the chip pattern 6, that is, the signal output level V.sub.1 of the chip pattern 6 is higher than the signal output level V.sub.2 of the surrounding surface when the reflection factor of the chip pattern 6 is higher than that of the surrounding surface. Detection of the line edges 7a, 7b, 7c of the chip pattern is accomplished by setting up a slice level V.sub.0 intermediately of the output levels V.sub.1 and V.sub.2 so provided and by making the points 13a, 13b, 13c whereat the output signal 12 slices the slice level V.sub.0 correspond to the line edges 7a, 7b, 7c of the chip pattern, thereby obtaining line edge position signals.
Now, such conventional method of measuring the line width of a chip pattern has suffered from two problems which will be mentioned below.
A first problem concerns the optical contrast of the chip pattern to be measured. In the process of forming an IC pattern on a semiconductor wafer, as is well-known, the wafer surface is subjected to various treatments and numerous layers of thin film are superposed upon one another. Thus, the contrast of the optical image of the chip pattern so provided is not uniform but varies under the influence of the treatments applied thereto. In the case as shown in FIG. 2 wherein the chip pattern signal is such that the contrast of the chip pattern is varied by the photoelectric conversion signal 12 and may only result in the level V.sub.1', even if the line width of the chip pattern on the wafer is invariable, measurement effected with the slice level fixed to V.sub.0 would cause errors to the line edge position signals 13d and 13e, in contradistinction with the case of a chip pattern wherein the photoelectric conversion signal has been at the level V.sub.1. To eliminate such errors, it is necessary to make the slice level V.sub.0 correspond to the variation in contrast of the chip pattern image by some method and re-set the slice level V.sub.0 by some method. Further, in the cases where the contrast of the chip pattern image is deteriorated to the extent that the photoelectric conversion signal is not sufficiently obtainable, the measurement itself could become impossible.
A second problem is that, in the example shown in FIGS. 1 and 2, the use of the microscopically projected image of the chip pattern to be measured causes the line edges of the chip pattern image to be affected by the optical transfer function (OTF) of the objective lens of the microscope. Therefore, to ensure more accurate measurement of the line edges, it is necessary to determine the slice level of the photoelectric conversion signal with the influence of the OTF of the objective lens taken into account; consequently it is extremely difficult to determine the line edges of the optically formed images of chip patterns which are formed under various conditions.